Changing a funding account is only possible when there are no pending transfers between your external funding account and your Qapital account, regardless of the direction of the transfer.
Pending transfers usually take 1-2 business days to clear, if you want to make sure you won't get additional pending transfers, you can always pause your account by going by going to the Transfers tab>Rules and toggle the Snooze button. Once you do that and after you see all your pending transactions settle you can try and change funding account again.